Conventional exhaust equipment members such as exhaust manifolds, turbine housings, etc. for automobile engines are made of heat-resistant cast iron such as Niresist cast iron (Ni—Cr—Cu-based, austenitic cast iron), heat-resistant, ferritic cast steel, etc. However, although the Niresist cast iron exhibits relatively high strength at an exhaust gas temperature up to 900° C., it has reduced oxidation resistance and thermal cracking resistance at temperatures exceeding 900° C., exhibiting poor heat resistance and durability. The heat-resistant, ferritic cast steel is utterly poor in strength at an exhaust gas temperature of 950° C. or higher.
Under such circumstances, JP2000-291430A proposes a thin exhaust equipment member formed by high-Cr, high-Ni, heat-resistant, austenitic cast steel, which is disposed at the outlet of an engine to improve the initial performance of an exhaust-gas-cleaning catalyst, at least part of paths brought into contact with an exhaust gas being as thin as 5 mm or less. Its weight loss by oxidation is 50 mg/cm2 or less when kept at 1010° C. for 200 hours in the air, 100 mg/cm2 or less when kept at 1050° C. for 200 hours in the air, and 200 mg/cm2 or less when kept at 1100° C. for 200 hours in the air. Its thermal fatigue life is 200 cycles or more when measured by a thermal fatigue test comprising heating and cooling at the heating temperature upper limit of 1000° C., a temperature amplitude of 800° C. or more, and a constraint ratio of 0.25, and 100 cycles or more when measured by a thermal fatigue test comprising heating and cooling at the heating temperature upper limit of 1000° C., a temperature amplitude of 800° C. or more, and a constraint ratio of 0.5. Accordingly, this exhaust equipment member has excellent durability when exposed to an exhaust gas at temperatures exceeding 1000° C., particularly around 1050° C., further around 1100° C.
The high-Cr, high-Ni, heat-resistant, austenitic cast steel forming the exhaust equipment member of JP2000-291430A has a composition comprising by mass 0.2-1.0% of C, 2% or less of Si, 2% or less of Mn, 0.04% or less of P, 0.05-0.25% of S, 20-30% of Cr, and 16-30% of Ni, the balance being Fe and inevitable impurities, which may further contain 1-4% of W and/or more than 1% and 4% or less of Nb.
From the aspect of environmental protection, automobile engines are recently required to have higher performance, increased fuel efficiency, and reduced exhaust gas emission. For this purpose, higher-power, higher-combustion-temperature engines are developed, elevating exhaust gas temperatures. Accordingly, exhaust equipment members are repeatedly heated and cooled in higher temperature regions than conventional ones. In addition, because they are directly exposed to a high-temperature exhaust gas from engines, they come to be used in severer oxidation environment.
When the exhaust equipment member is exposed to a high-temperature exhaust gas containing oxides such as sulfur oxide, nitrogen oxide, etc., or to the air when heated to high temperatures, an oxide layer is formed on its surface. The thermal expansion difference between the oxide layer and the equipment member matrix, etc. cause microcracks to generate from the oxide layer, through which an exhaust gas intrudes into the equipment member, resulting in further oxidation and cracking. The repetition of oxidation and cracking causes further cracking, resulting in cracks penetrating into the equipment member. The oxide layer peeling from the equipment member may contaminate a catalyst, etc., and cause the breakage and trouble of turbine blades in a turbocharger, etc. Accordingly, the exhaust equipment members exposed to a high-temperature exhaust gas containing oxides are required to have high oxidation resistance.
For higher power and higher-temperature combustion, the so-called direct-injection engine with a combustion chamber, into which gasoline is directly injected, has become widely used for automobiles. Because gasoline is introduced from a fuel tank directly into combustion chamber in the direct-injection engine, only a small amount of gasoline leaks even in the collision of the automobile, making large accident unlikely. Accordingly, instead of disposing exhaust equipment members such as an exhaust manifold, a turbine housing, etc. forward, and intake parts such as an intake manifold, a collector, etc. rearward, intake parts are conventionally disposed in front of an engine to introduce a cold air into a combustion chamber, while exhaust equipment members directly connected to an exhaust-gas-cleaning apparatus are disposed on the rear side of an engine to quickly heat and activate the exhaust-gas-cleaning catalyst at the start of the engine. However, because the exhaust equipment members disposed on the rear side of the engine are unlikely subjected to air flow during driving, resulting in higher surface temperature, they are required to have improved heat resistance and durability at high temperatures.
From the aspect of environmental protection, the exhaust-gas-cleaning catalyst should be heated and activated at the start of the engine. Accordingly, the temperature decrease of the exhaust gas passing through the exhaust equipment members should be suppressed. To suppress the exhaust gas temperature from decreasing (to avoid heat from being removed from the exhaust gas), the exhaust equipment members should have as small heat mass as possible, so that they should be thin. However, because thinner exhaust equipment members are more likely subjected to temperature elevation by the exhaust gas, they should have excellent heat resistance and durability at high temperatures.
Thus, the exhaust equipment members for automobile engines should cope with higher temperatures, severer operation conditions, etc., for instance, exhaust gas temperature elevation and oxidation, surface temperature elevation caused by disposing them rearward, temperature elevation caused by making them thinner. Specifically, the exhaust equipment members are likely to be exposed to a high-temperature exhaust gas at 1000-1150° C., and the exhaust equipment members per se exposed to such high-temperature exhaust gas are heated to 950-1100° C. Accordingly, the exhaust equipment members are required to have high heat resistance and durability and a long life at such high temperatures. To meet this demand, materials forming the exhaust equipment members should also have excellent high-temperature strength, oxidation resistance, ductility, thermal cracking resistance, etc.
With respect to the high-temperature strength, the exhaust equipment members should have not only high high-temperature tensile strength, but also high high-temperature yield strength, strength for suppressing thermal deformation (plastic deformation by compression) against compression stress generated under constrained conditions at high temperatures. Accordingly, the high-temperature strength is represented by high-temperature yield strength and high temperature tensile strength.
With respect to the oxidation resistance, it is necessary to suppress the formation of oxide layers acting as the starting points of cracking even when exposed to a high-temperature exhaust gas containing oxides. The oxidation resistance is represented by weight loss by oxidation. The exhaust equipment members are cooled from high temperatures to an ambient temperature by the stop of engines, and during the cooling process, compression stress generated at high temperatures is turned to tensile stress. Because the tensile stress during the cooling process causes cracking and breakage, the exhaust equipment members should have such ductility as to suppress the generation of cracking and breakage at room temperature. Accordingly, the ductility is represented by room-temperature elongation.
Thermal cracking resistance is a parameter for expressing these high-temperature strength, oxidation resistance and ductility as a whole. The thermal cracking resistance is represented by a thermal fatigue life [the number of cycles until thermal fatigue fracture occurs by cracking and breakage caused by the repetition of operation (heating) and stop (cooling)].
The exhaust equipment members are subjected to mechanical vibration, shock, etc. during the production process and assembling to engines, at the start of or during the driving of automobiles, etc. The exhaust equipment members are also required to have sufficient room-temperature elongation to prevent cracking and breakage against outside forces generated by these mechanical vibration and shock.
The exhaust equipment member disclosed by JP2000-291430A is particularly excellent in oxidation resistance, but recent demand to higher performance requires further improvement in thermal fatigue life and room-temperature elongation when exposed to an exhaust gas at 1000° C. or higher.